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Climate and ENSO Associations with Malaria Incidence in Colombia


Germán Poveda
Postgrado en Aprovechamiento de Recursos Hidráulicos,
Universidad Nacional de Colombia
Medellín, Colombia


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More than five million people live in malaria-prone regions of Colombia. During 1996, transmission of malaria reached 42 cases per 1,000 inhabitants in high-risk areas. The province of Chocó (along the Pacific coast) experienced more than 80,000 cases during 1998 when the population at risk was of 380,000. Malaria is transmitted by mosquito vectors such Anopheles albimanus, A. darlingi and A. nuñeztovari, transmitting Plasmodium falciparum (46.5%) and P. vivax (53.5%). Climate and its variability constitute important factors to explain the incidence of malaria in low-land tropical endemic areas of Colombia. During "normal years", endemic malaria in those regions exhibit a clear-cut "normal" annual cycle, tightly associated with prevalent climatic conditions, mainly mean temperature, precipitation, humidity and river discharges In general, during dry seasons malaria incidence increases concomitant and following periods of reduced rainfall, and river flows, and augmented average and minimum air temperature. (Poveda et al., 2001).

figure 1
ENSO is the main forcing mechanism of Colombia's hydro-climatology at interannual timescales. Overall, Colombia experience droughts--reduced rainfall, soil moisture, river discharges, and evaporation--along with increase in air temperature during El Niño events. The reverse is generally valid for La Niña. The impact of ENSO occurs earlier and stronger in western and central Colombia than in the east. Seasonally, the impacts of ENSO are more pronounced during December-January-February (year +1)1, September-October-November (year 0) and June-July-August (year 0), in that order. The March-April-May seasons in both years 0 and +1 are the least affected (Poveda et al., 2001b).

During historical El Niño events (interannual time scale) there is an intensification in the incidence of malaria in Colombia, as reported by Poveda and Rojas (1995), and Poveda et al. (2000). See Figure 1.

figure 2
The "normal" annual cycle of malaria is intensified during El Niño events, at local, regional and national levels. Figure 2 shows the spatial distribution of average annual cycle (black bars) and during El Niño events (red bars), for P. vivax malaria recorded at 16 localities on the lowlands of the northwestern province of Antioquia. Figure 2 shows the annual cycle considered from the June (year 0) to May (year +1). Figure 2 confirms that malaria cases intensify during El Niño at local level, considering a "climate-malaria" year from the second semester of year 0 to the first semester of year +1. This result is also found in multiple towns in rural Colombia (not shown), and also in malaria-prone regions nationwide.

These malaria outbreaks are associated with a consistent pattern of hydrological and climatic anomalies occurring during El Niño in Colombia: increase in mean temperature, decrease in precipitation, increase in dew point, and decrease in river discharges, all of them favoring malaria transmission. There is a coupling between the "normal" annual and the interannual patterns of malaria incidence in Colombia. Such coupling explains why the effect appears stronger and more persistent during the second half of El Niño's year (0), and during the first half of year (+1) (Poveda et al., 2001). The identified coupling between annual and interannual timescales in the climate-malaria system shed new light towards understanding the exact linkages between environmental, entomological and epidemiological factors conductive to malaria outbreaks, and also imposes the coupling of those timescales in public health intervention programs in Colombia. We are currently developing modeling and fieldwork to better understand and explain the observed linkages between climate, ENSO and malaria incidence in Colombia.
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1Note on notation: "Year 0" refers to the calendar year (January to December) when an El Niño or La Niña event develops; "year +1" refers to the calendar year after the year an event develops.
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Links

References

  • Poveda, G., and Rojas, W., Impact of El Niño phenomenon on malaria outbreaks in Colombia (in Spanish). Proceedings XII Colombian Hydrological Meeting, Colombian Society of Engineers, Bogotá, 647–654. 1996.
  • Poveda, G., N. E. Graham, P. R. Epstein, W. Rojas, M. L. Quiñonez, I. D. Vélez, y W. J. M. Martens, Climate and ENSO Variability Associated with Vector-Borne Diseases in Colombia. In: Diaz, H.F. and V. Markgraf (Eds.), El Niño and the Southern Oscillation, Multiscale Variability and Global and Regional Impacts. Cambridge University Press, pp. 183-204. 2000.
  • Poveda, G., W. Rojas, I. D. Vélez, M. Quiñones, R. I. Mantilla, D. Ruiz, J. Zuluaga, and G. Rua, Coupling between Annual and ENSO timescales in the Malaria-Climate association in Colombia. Environmental Health Perspectives, 109, 489-493. 2001a.
  • Poveda, G., A. Jaramillo, M. M. Gil, N. Quiceno, and R. Mantilla. Seasonality in ENSO Related Precipitation, River Discharges, Soil Moisture, and Vegetation Index (NDVI) in Colombia. Water Resources Research, 37(8), 2169-2178. 2001b.